The invention will be described by way of example in the context of protecting equipment from potentially harmful transients, such as those caused by lightning strikes. It should be appreciated that this description with reference to protection from lightning produced transients is by way of example only and that embodiments of the invention may be used for purposes other than this specific application.
FIG. 1 depicts an item of electronic equipment 6, for example a telephone switch or a radio transceiver, which is susceptible to damage from over-voltage transients on lines 2 and 4. Equipment 6 is earthed at point 10 by means of an earthing network that will usually have an associated earthing impedance 8. Impedance 8 is typically inductive but might also be resistive or capacitive in nature.
It is common practice to protect equipment 6 by means of a protective module which is inserted in series between lines 2 and 4 and equipment 6. Such modules typically include high voltage breakdown components such as gas discharge tubes (GDTs) and metal oxide varistors (MOVs) for connection between the lines and ground. In the event of a lightning strike or other transient occurring on lines 2 and 4 then the protective module shunts the transient to ground thereby protecting the equipment. However a problem is associated with such prior art modules as will now be described with reference to a typical prior art protection module 24, shown in FIG. 2.
With reference to FIG. 2, protection module 24 includes first and second input terminals 12 and 14 and first and second output terminals 26 and 28 for connecting the module in series between lines 2 and 4 and equipment 6 as shown in FIG. 3. Protection modules configured to protect only a single line, or more than two lines are also available. Internally, protection module 24 includes a first limiting resistor 18 connected between first input terminal 12 and first output terminal 26. A MOV 30 is connected between first output terminal 26 and a protection earth reference terminal 22. Similarly, a further limiting resistor 20 is connected between second input terminal 14 and second output terminal 28. A second MOV 32 is connected between second output terminal 28 and protection earth terminal reference 22. Finally, a gas arrestor in the form of gas discharge tube (GDT) 16 is connected from first and second input terminals 12 and 14 to ground reference terminal 22.
FIG. 3 shows protection module 24 in use wherein protection module 24 and equipment 6 are bound to an equipotential earth 37 by connections that exhibit certain levels of earthing impedance which may be a combination of resistance, capacitance and inductance. For purposes of explanation the earthing impedances are represented by inductors 36 and 34. As previously mentioned, where protection module 24 and equipment 6 are not adequately or correctly bound to the system earth potential, earthing impedances 36 and 34 may be substantial.
Equipotential earth 37 is connected to a true or real earth 40 by an earthing conductor that exhibits an impedance represented by inductor 38.
Where the earthing connections are good, so that impedance 36 is negligible, then in the event of a transient occurring on lines 2 and/or 4, GDT 16 and MOVs 30 and 32 provide a measure of protection for equipment 6. However, if the earth connections are not good, or the cables used to provide these connections are relatively long, then impedance 36 is likely be significant. In that case should transient over-voltages occur on lines 2 and 4 then protective module 24 will react by shunting the associated current to ground via MOVs 30 and 32 and GDT 16. However as current commences to be shunted to ground impedance 36 will cause a rise in the protection earth reference voltage at terminal 22. In conventional protection systems, such as that provided by module 24, the rise in the protection earth reference voltage is unavoidably reflected through terminals 26 and 28 onto terminating equipment 6. Consequently a transient current flows through equipment 6 which may cause damage.
It will be realised that in the prior art circuit of FIG. 2, the point at which the internal shunt components, MOVs 30 and 32 and GDT 16, commence operation is dependent on the voltages on lines 2 and 4 relative to the protection earth reference voltage at terminal 22. If the protection earth reference rises in voltage, for whatever reason including the one identified above, then that rise will be reflected on to terminating equipment 6. That is, if a transient on lines 2 and 4 results in damage to equipment 6 then all that may be discerned is that the equipment has not been adequately protected. There is no way of knowing if the failure to protect the equipment was due to poor earthing, the inadequacy of the shunt components within module 24 or because the surge or transient was applied directly to equipment 6 and bypassed protection module 24.
From the above discussion it will be understood that there is a problem with prior art protective apparatus of the type typified by the module of FIG. 2. Namely, protective modules of the type of FIG. 2 do not provide adequate protection in the case of a rise in the protection earth reference voltage. A further problem is that when such a rise occurs and a transient causes damage to the terminating equipment, considerable investigative effort is then required to determine the cause of the damage.
It is an object of the present invention to provide a protection apparatus that addresses one or more of the problems referred to above.